HIV-1 subtype C (HIV-1C) is the dominant HIV-1 subtype in the AIDS epidemic. Early virological and immunological events in acute HIV-1C infection are not well understood. A minimal goal for efficacy with a vaccine candidate that is expected to give non-sterilizing immunity would be to reduce the amount of HIV replication during the acute stage of infection. Understanding associations between virological and immunological parameters in acute HIV-1C infection will be critical from the perspective of HIV vaccine design and for the selection of pertinent HIV vaccine trial endpoints. The purpose of the current pilot proposal is to identify individuals as early as possible in HIV-1C infection and characterize viral and immune parameters in acute HIV-1C infection. This exploratory R21 study is designed as an extension to the HIV Vaccine Preparedness Study (HVTN Protocol 903), a prospective cohort study of 500 uninfected individuals at high risk for HIV infection to start in Botswana in early 2003. This study will utilize the epidemiologic and laboratory infrastructure in place in Botswana, a country in southern Africa with an estimated HIV prevalence of about 35-40%. A more comprehensive RO1 application to investigate acute HIV-1C infection will be submitted when appropriate. There are two Specific Aims in this study. The first Specific Aim of this study is to analyze potential trends between viral load (both plasma and proviral) and viral genetic diversity in acute HIV-1C infection. These data should indicate a potential relationship between virological parameters in acute HIV-1C infection and viral set point. Virological methods will include DNA and RNA isolation, plasma viral load and cell-associated proviral load quantification, PCR amplification, cloning, and sequencing. The second Specific Aim targets the characterization of cellular immune responses in acute HIV-1C infection and the identification of immunological parameters that correlate with viral set point. The magnitude, breadth, and diversity of virus-specific CD4+ and CD8+ T-cell immune responses in acute HIV-1Cinfection will be analyzed. Immunological methods will include IFN-gamma-ELISPOT assay and intracellular cytokine staining using the four-color FACSCalibur flow cytometer.